In Patent Literature 1, there is a description of causing a faint high-frequency open-phase current to flow to a winding of a motor to warm the motor winding when the amount of a liquid refrigerant retained in a compressor reaches a predetermined value or higher. Accordingly, breakage failure of the compressor is prevented by preventing liquid compression due to starting of an operation in a state where a liquid refrigerant is retained in the compressor.
In Patent Literature 2, there is a description of controlling an on/off cycle of a switching element to periodically reverse a direction of a current flowing to a stator coil of a motor. With this technique, not only heat generation due to ohmic loss but also heat generation due to hysteresis loss is performed so that sufficient preheating can be performed with a less consumption current, thereby improving the power efficiency.